diff --git a/tests/testthat/test_preprocess_ww_data.R b/tests/testthat/test_preprocess_ww_data.R
index 2b0b7d5b..5e60f2b6 100644
--- a/tests/testthat/test_preprocess_ww_data.R
+++ b/tests/testthat/test_preprocess_ww_data.R
@@ -381,12 +381,15 @@ test_that("Function handles LOD values equal to concentration values", {
 })
 
 test_that("Constant population per site", {
-  wrong_pop <- ww_data
-  ww_data$site_pop <- 1e6 + seq_len(nrow(ww_data))
+  wrong_pop <- cbind(
+    ww_data,
+    ww_data |> dplyr::mutate(site = max(.data$site) + 1)
+  )
+  wrong_pop$site_pop <- 1e6 + seq_len(nrow(ww_data))
 
   expect_error(
     preprocess_ww_data(
-      ww_data,
+      wrong_pop,
       conc_col_name = "conc",
       lod_col_name = "lod"
     ),